Optical fiber lasers and amplifiers can provide a flexible, rugged and simple source of light. It is often desirable that an optical fiber laser or amplifier can provide light having a selected polarization. Polarization refers to the direction of an electric field vector of a component of a propagating wave, and the selected polarization is typically specified as being present to a selected degree (e.g., a linear polarization can be specified as being present so as to have a selected polarization ratio to an orthogonal linear polarization). It can also be desired that the light provided be of high quality, that is, have one or more of low divergence, high coherence, a narrow spectrum, a narrow beam diameter and a low M2 parameter.
Light having a selected polarization can be useful in a variety of circumstances. For example, a particular fiber laser or amplifier may not operate over the wide range of wavelengths available from other types of lasers or amplifiers. Many of the applications for which such light sources are used, such as medical, materials processing or defense applications, require a particular wavelength of light. It can be advantageous to convert the light from such a fiber laser or amplifier to a different wavelength. One of the more useful processes for converting light to a different wavelength, namely, launching light on to a crystal, can require that the light have a particular polarization. As another example, lithium niobate modulators, often used to impress information on light, also work most efficiently when provided with light having a selected polarization. U.S. Published Patent Applications US 2002/0172486 A1, entitled “Single Polarization High Power Fiber Lasers and Amplifiers”, published Sep. 21, 2002; US 2002/0159139 A1, entitled “Polarization-Maintaining Optical Fiber Amplifier Employing Externally Applied Stress-Induced Birefringence”, published Oct. 31, 2002 (now U.S. Pat. No. 6,724,528); and US 2003/0086668 A1, entitled “Linearly Polarized Fiber Amplifier”, published May 8, 2003 (now U.S. Pat. No. 6,825,974), discuss various techniques for obtaining light having a selected polarization from fiber lasers and amplifiers. See also U.S. Pat. No. 6,072,811, entitled “Integrated Passively Modelocked Fiber Lasers and Method For Constructing the Same”, issued Jun. 6, 2000.
In addition to the patents and applications noted above, see, for example, U.S. Published Patent Application US 2003/0123494 A1, entitled “Single Polarization Fiber Laser”, published Jul. 3, 2003, which teaches fabricating a Bragg grating in a fiber to introduce a differential loss between two polarizations. See also U.S. Pat. No. 5,511,083, entitled “Polarized Fiber Laser Source”, issued Apr. 23, 1996, which teaches a similar technique. The use of polarizing elements external to a fiber is also known. See, for example, U.S. Pat. No. 6,049,415, entitled “Polarization Maintaining Fiber Lasers and Amplifiers”, issued Apr. 11, 2000.
In certain circumstances, available optical fiber lasers and amplifiers do not readily provide high enough power output. The above noted process for converting wavelength is not always efficient, and higher power can be required. Also, fiber lasers and amplifiers typically do not readily produce as much power as other types of lasers or laser amplifiers, such as bulk solid state devices, gas lasers, or diode lasers, which have gained acceptance in industrial settings such as welding and the like, in large measure due to the ability to provide high enough power. Accordingly, the number of applications for which fiber lasers can be used is unduly limited, despite their many advantages.
Nonlinear processes, such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are responsible, in large part, for limiting the power output of fiber lasers and amplifiers. Though these processes are complex, each can typically be reduced, at least in part, by limiting the power density in the core of the fiber. The power density can be reduced by increasing the diameter of the core of the fiber and/or reducing the numerical aperture (NA) of the core, such that the fiber has a larger mode field diameter. Essentially, the power of the light propagating along the fiber is more spread out, such that the power density is reduced. Unfortunately, fibers having large core diameters can support multiple spatial (transverse) modes. The presence of such modes tends to degrade the quality of the light provided by the fiber. U.S. Pat. No. 6,496,301, issued on Dec. 17, 2002 to Koplow, Kliner and Goldberg, involves positioning a fiber, which can have a large core and a low NA, to substantially attenuate, via bend loss, higher order modes such that a fiber amplifier provides light having a lower M2 parameter that would be provided absent the positioning of the fiber.
Existing techniques, however, can be considered unduly complex or expensive and impractical in certain applications.
Accordingly, it is an object of the present invention to address one or more of the deficiencies or drawbacks of the prior art.